Severe combined immunodeficiency X-linked T-cell-negative/B-cell-positive/NK-cell-negative (XSCID) [MIM:300400]: A form of severe combined immunodeficiency (SCID), a genetically and clinically heterogeneous group of rare congenital disorders characterized by impairment of both humoral and cell-mediated immunity, leukopenia, and low or absent antibody levels. Patients present in infancy recurrent, persistent infections by opportunistic organisms. The common characteristic of all types of SCID is absence of T-cell-mediated cellular immunity due to a defect in T-cell development. {ECO:0000269|PubMed:7557965, ECO:0000269|PubMed:7668284, ECO:0000269|PubMed:7860773, ECO:0000269|PubMed:7937790, ECO:0000269|PubMed:8027558, ECO:0000269|PubMed:8088810, ECO:0000269|PubMed:8299698, ECO:0000269|PubMed:8401490, ECO:0000269|PubMed:8900089, ECO:0000269|PubMed:9049783, ECO:0000269|PubMed:9150740}. Note=The disease is caused by mutations affecting the gene represented in this entry.; X-linked combined immunodeficiency (XCID) [MIM:312863]: Less severe form of X-linked immunodeficiency with a less severe degree of deficiency in cellular and humoral immunity than that seen in XSCID. {ECO:0000269|PubMed:7883965, ECO:0000269|PubMed:9399950}. Note=The disease is caused by mutations affecting the gene represented in this entry.